1. Field of the Invention
The present invention relates generally to an emissions reduction system employed in an exhaust passage of an internal combustion engine, and more particularly to a control system for optimizing the reduction of nitrogen oxides in exhaust gas produced by an internal combustion engine employing a lean air-fuel ratio.
2. Background and Summary of the Invention
Increasingly stringent government regulations for the allowable emission levels of carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) have resulted in the use of catalytic converters on most passenger vehicles sold in the United States. The task of the catalytic converter is to promote chemical reactions for the conversion of these pollutants to carbon dioxide, water, and nitrogen.
For automotive exhaust applications, the pollutant removal reactions are the oxidation of carbon monoxide and hydrocarbons and the reduction of nitrogen oxides.
Converters are of two basic catalyst types: the two-way converter (oxidation) and the three-way converter (oxidation and reduction). Both types typically employ either a pellet or monolith design.
The two-way catalytic converter is placed in the exhaust system between the exhaust manifold and muffler. When the hot gases are forced through the converter, they contact the catalyst-coated pellets or honeycomb, depending on the type. The resulting exothermic reaction cataylzed by the catalyst causes a rapid increase in the exhaust temperature. This, in turn, causes the carbon monoxide and hydrocarbons to change (by means of an oxidizing process) into water (H2O) vapor and carbon dioxide (CO2) gas. The two-way oxidizing converter does not reduce the nitrogen oxides (NOx).
The three-way converter uses an additional catalyst bed typically coated with platinum, palladium, rhodium, and combinations thereof. The three-way converter is capable of removing all three pollutants (i.e., carbon monoxide, hydrocarbons, and nitrogen oxides) simultaneously, provided that the catalyst is maintained in a chemically correct environment that is neither overly oxidizing or reducing.
Although catalytic converters work well with engines employing a stoichiometric air-fuel ratio (i.e., 14.7:1), they are not as effective with engines employing a lean air-fuel ratio (i.e., 16.0:1 or higher). Examples of engines exhibiting lean burn operation are diesel and certain newer generation gasoline engines (e.g., direct injection gasoline engines).
A type of catalyst for removing NOx from the exhaust gas of internal combustion engines during lean burn operation, often called a xe2x80x9cNOx trapxe2x80x9d or xe2x80x9cNOx absorber,xe2x80x9d is disclosed in U.S. Pat. No. 5,404,719 issued Apr. 11, 1995. This catalyst generally comprises alkaline metals or alkaline earth materials like potassium or strontium in combination with a precious metal like platinum. Under conditions of excess oxygen, i.e., when the exhaust gas is lean, this trap is capable of storing/absorbing, nitrogen oxides. When the oxygen concentration of the exhaust gas is lowered, the NOx is released from the NOx trap catalyst. These traps thus operate in a different way compared to conventional lean-burn catalysts. More particularly, the widely held mechanism for NOx trap operation is that the precious metal first oxidizes NO to NO2 and the NO2 subsequently forms a nitrate complex with the alkaline material. In a stoichiometric or rich environment, the nitrate is thermodynamically unstable, and the stored NOx is released. NOx then catalytically reacts with excess reducing species in the exhaust gas to form N2.
Another catalyst system for removing NOx from exhaust gases of lean burn internal combustion engines is referred to as the Selective Catalytic Reduction (SCR) system. The SCR catalyst system uses an additive, such as urea, which is introduced into the exhaust stream wherein it combines with the NOx over a suitable catalyst to eliminate the NOx from the tailpipe emissions. Urea decomposes in the heat of the exhaust stream into ammonia which reacts with the NOx. Ammonia itself could be introduced but is much more dangerous to have onboard the vehicle. The reaction between the ammonia and the NOx over the catalyst is highly temperature dependent. If too much urea is metered into the exhaust system and there is either insufficient NOx in the exhaust stream or the catalyst temperature is too low to promote efficient conversion, then ammonia gas will exit the tailpipe as a dangerous and foul smelling gas. Therefore, the proper control of the quantity of urea injected is very important as well as the temperature of the NOx catalyst system.
One catalyst system that utilizes a SCR system is marketed under the tradename SINOX(trademark) by Siemens Automotive Corporation (Auburn Hills, Mich.). The SINOX(trademark) system""s electronic control system processes temperature and emission level information fed from sensors, and then meters and injects appropriate amounts of urea into the catalyst system. A reduction of the NOx levels of up to 70% is claimed by the manufacturer.
However, the aforementioned methods of removing NOx from the exhaust gas of a lean burn internal combustion engine have failed to achieve an optimal level of NOx reduction. In particular, previous methods have not achieved the proper control and maintenance of the temperature of the NOx catalyst system within an optimal temperature range. Accordingly, the level of NOx exiting the tailpipe is relatively high when the automobile is initially started and the NOx catalyst system is relatively cool. The level of NOx exiting the tailpipe decreases gradually as the NOx catalyst system warms up to operating temperatures, thus becoming more efficient. However, in the interim, unnecessarily high amounts of NOx have exited the tailpipe into the atmosphere, thus contributing to pollution concerns.
Therefore, there exists a need for a control system for optimizing the removal of nitrogen oxides from the exhaust gas of a lean burn internal combustion engine.
In accordance with one aspect of the present invention, a control system for use in an internal combustion engine automobile having a NOx catalyst system, comprises a temperature control assembly placed upstream of the NOx catalyst system, the temperature control assembly selectively adjusting the temperature of the exhaust gas prior to the exhaust gas being introduced into the NOx catalyst system.
In accordance with another aspect of the present invention, a control system for use in an internal combustion engine automobile having a NOx catalyst system, comprises a temperature control assembly placed upstream of the NOx catalyst system, the temperature control assembly selectively adjusting the temperature of the exhaust gas prior to the exhaust gas being introduced into the NOx catalyst system. A temperature sensor is placed downstream of the temperature control assembly and upstream of the NOx catalyst system, the first temperature sensor detecting the temperature of the exhaust gas prior to introduction into the NOx catalyst system.
In accordance with yet another aspect of the present invention, a control system for use in an internal combustion engine automobile having a NOx catalyst system, comprises a temperature control assembly placed upstream of the NOx catalyst system, the temperature control assembly selectively adjusting the temperature of the exhaust gas prior to the exhaust gas being introduced into the NOx catalyst system. A NOx sensor is placed downstream of the temperature control assembly and upstream of the NOx catalyst system, the NOx sensor sensing the level of NOx in the exhaust gas. A first temperature sensor is placed downstream of the temperature control assembly and upstream of the NOx catalyst system, the first temperature sensor detecting the temperature of the exhaust gas prior to introduction into the NOx catalyst system. A second temperature sensor is placed downstream of the NOx catalyst system, the second temperature sensor detecting the temperature of the exhaust gas after exiting the NOx catalyst system.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood however that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description.